The present invention relates to a semiconductor device and method for manufacturing the same, and more specifically, to a semiconductor device in which moisture-proofness of an airtightly sealed semiconductor package is improved and a method for manufacturing the same.
In a semiconductor chip such as an IC, LSI or VLSI, a highly-integrated circuit surface tends to be affected by the change in ambient temperature, humidity, or fine dust. Further, since the semiconductor chip itself may be broken by mechanical vibration or other type of shock, a semiconductor device includes a package for covering the semiconductor chip.
The package may be either an airtight seal type package or a resin mold type package. In the airtight seal type, an air-tight space is formed in the central portion of a package member, and the semiconductor chip is arranged within the airtight space. Ceramics having a high moisture-proofness are generally used as the hollow package material, but synthetic resins which have low cost and are easy to work with are also used.
In a semiconductor device using a hollow package made of synthetic resin, a semiconductor chip is secured to a bonding surface (a chip bonding surface) of a hollow portion with adhesive and is electrically connected to a lead frame with a bonding wire. Opposite ends of the lead frame are opened to an inner side and an outer side of the package.
The top surface of the hollow package is airtightly sealed by securing a cover member or lid such as a transparent or opaque synthetic resin plate, a glass plate or the like thereto with adhesives.
When a hollow package made of synthetic resin is used, there arises a problem in that even if the cover member is secured with adhesive, a fine amount of moisture enters the package as time passes. As a result, electrodes of the semiconductor chip or internal circuit corrode and degrade the performance of the semiconductor chip, and finally result in the semiconductor chip being unusable.
Even if a careful seal is applied to an adhesive portion of the cover or a seal portion of the lead frame which are believed to be where the moisture enters, it is still not possible to prevent entry of moisture after passage of time.
The present inventors have studied the entry of moisture into airtightly sealed semiconductor packages. Originally, it was assumed that a route of entry of moisture into the semiconductor package was via the adhesive portion of the cover member or the seal portion of the lead frame, and tests were repeatedly performed to verify this assumption.
However, it has been determined that the entry of moisture from the adhesive portion of the cover member or the seal portion of the lead frame possess is not much of a problem since various countermeasures may be taken to avoid this. The entry of moisture into the semiconductor package principally results from the moisture which transmits through a molded body from the lowermost surface of the package, that is, the bottom surface of the hollow package.
It is to be noted that the entry of moisture into the semiconductor package was tested in the following manner. A package sealed by a transparent cover member was placed into a commercially available pressure cooker tester (PCT) and heated and pressurized at 121.degree. C., a relative humidity of 100% and a gauge pressure of 1 kg/cm.sup.2 for a predetermined number of hours, after which the package was removed to examine if condensation had formed caused by moisture which entered internally via a transparent cover member under normal temperature.
Those packages which had no condensation were further heated and pressurized for the predetermined number of hours until condensation appeared.
In the present invention, the quality of the moisture proofness of the package was determined by the heating and pressurizing time until condensation appeared in the inside of the transparent cover member.
It is an object of the present invention to provide a semiconductor device of an airtight seal type having excellent moisture-proofness which can effectively prevent the entry of moisture into the semiconductor device.